RFID technology allows identification data to be collected remotely, which provides a significant advantage in identifying articles, parcels or other items. To access identification data stored in an RFID transponder (commonly referred to as a “RFID tag” or “tag”), an RFID reader/encoder generates an energy field via a transmission beam to interrogate the RFID tag, and subsequently, to retrieve data stored in the RFID tag. The data received from the RFID tag is processed by a computer system to identify the item that is associated with the RFID tag. Due to its convenience and reliability, RFID technology has found a wide range of applications, including item tracking, item location, inventory assessment, etc.
However, complications may occur in the detection of RFID tags in monitored areas where reverberated space present fading and multipath affects that impede reception of responsive signals from RFID tags. In an attempt to overcome these reception challenges, conventional RFID systems utilize a circular polarized antenna. However, circular polarized antennas incur a mismatch loss of 3 dB in each transmission direction (reader-to-tag and tag-to-reader) or as much as 6 dB per transmission/reception. As a simple example, consider FIG. 1, which illustrates a RFID reader 100 transmitting an interrogation signal to an RIFD tag 102. Along a direct transmission path 104, the circular polarized interrogation signal includes a vertical component 106 and a horizontal component 108 and has a clockwise polarization (indicated by arrow 110). Due to the finite beam width of the antenna of the RFID reader 100, a divergent transmission path 112 also exists (only one divergent path illustrated for simplicity) which includes a vertical component 106′ and a horizontal component 108′ and has a clockwise polarization (indicated by arrow 110′). Upon contact with a reflective (conductive) surface 114, a reflective transmission path 116 is created. As can be seen in the illustration of FIG. 1, the reflected interrogation signal has a vertical component 106″ and horizontal component 108″ that has a reduced magnitude due to energy lost in the reflection. Also, the horizontal component 108″ has incurred a phase shift of 180° resulting in the reflected interrogation signal being polarized in a counter-clockwise direction (as indicated by the arrow 110″). As this signals move toward the RFID tag 102, there is a possibility that they will collide at or near the antenna of the RFID tag 102. If this occurs, the vertical components 106, 106″ will undergo constructive interference (i.e., add) and the horizontal components 108, 108″ will undergo destructive interference (i.e., subtract). Such changes the polarization of the received interrogation signal 118 to be elliptically polarized possessing a particular mean polar angle (from the point of view of the RFID tag 102). This presents a mismatch that may greatly reduce the energy available to energize (or power) the RFID tag 102 to transmit a responsive signal. If the RFID tag can derive sufficient power from the RF carrier of the interrogation signal, inter-symbol interference resulting from the collision may distort the baseband component of the interrogation signal causing the RFID tag not to respond. However, it will be appreciated that if the RFID tag 102 is activated to transmit a responsive signal, similar transmission impediments occur in the responsive transmission direction.
Moreover, if items associated with RFID tags are randomly placed and oriented within the monitored area, the RFID tags (and the respective antennas) will be randomly oriented with respect to the RFID readers. Random orientation may also promote polarization errors and other deficiencies in the responsive signals returned from the RFID tags.
Accordingly, there is a need for a method and apparatus for improving reception of responsive signals from RFID tags.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the disclosure.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.